Many articles of manufacture employing heat seals are currently available in the marketplace. Generally, the seals on such articles may be employed by welding two separate portions of the article together. For example, plastic parts usefully employed in machines and toys may be constructed by joining together two individual plastic pieces by heating one or both of the plastic pieces, pressing them together, and then, allowing them to cool. Specifically, heat sealing is very important in packaging applications. Packages formed by a heat seal provide for the efficient transportation of a consumer item within the package, provide a display of the consumer item that promotes sales, and, in the food industry, the packaging is employed to preserve the freshness of the consumer item. Most importantly and related to the actual heat sealing process, a manufacturer of such packages or any other like article requiring a seal requires excellent processability in terms of strong seals formed at low sealing temperatures.
Various types of polymers are used to form articles, which include packages, that may be joined together or sealed by the application of heat and/or pressure. Polymers or blends of polymers used to make the articles are selected for use because they provide a strong seal, which is easily and rapidly formed by a single short application of heat and/or pressure. Occasionally, the entire heat sealed article is constructed from the same polymer or a blend of polymers or by the coextrusion of the same or different polymers.
More often, the article is constructed of various areas or layers of different materials, and polymers which provide good heat sealing properties are utilized only in areas, or layers, where heat sealing will ultimately be necessary. This type of construction is employed because the articles, for instance multilayer films, should have desirable physical and mechanical properties such as clarity, strength, resistance to puncture and tearing, in addition to heat sealing properties, and should be easily processed by high speed equipment. Many plastic materials are known to possess good physical and mechanical properties but often do not also possess good heat sealing properties. For example, polypropylene has good strength and clarity and is resistant to tearing, but does not readily form good seals at the temperatures which are preferred in commercial sealing machinery. Conversely, some polymers with good heat sealing properties do not have adequate strength or clarity.
The packaging art has therefore developed multiple layer articles such as multilayer films incorporating one or more layers of the same or different types of polymers blended or coextruded together that provide good mechanical and physical properties and providing one or more additional layers formed from polymers that provide the article of manufacture with good heat sealing properties. In this way, for example, a film may be produced having a substrate layer of polypropylene provided for strength and clarity, and a layer of polyethylene to provide good heat sealing properties. Other articles, in addition to films, may be similarly constructed with a plurality of materials, each material selected to contribute to one or more of the desired properties of the final article.
Ionomers are tough, durable, transparent thermoplastics that are widely used in the form of films, molded components, extruded products and foams for packaging and a wide range of consumer and industrial products. In the area of packaging, particularly food packaging films constitute the largest single market for ionomers. Ionomers have excellent heat sealing characteristics, exhibiting high hot tack and heat seal strengths at low sealing temperatures. It is widely used as a heat-seal layer in a variety of flexible packaging composites produced by coextrusion, extrusion coating, lamination, and combinations of these techniques. Application examples including packaging for meat, cheese, snack foods and pharmaceuticals.
Polymers should be reasonably compatible to have good heat sealing performance when sealed to each other. Forming effective, strong seals generally requires a partial melting of the two films being sealed together and some intermixing of these melted films at the seal interface. This intermixing is important in hot tack strength where the seal strength is measured while the seal is still in a partially molten state. However, ionomers, due to their strong ionic/polar functionality, are not particularly compatible with conventional Ziegler-Natta produced linear polyethylenes. Therefore, a need exists in the industry for a film that effectively heat seals with ionomers.